Adjustable wave guide termination



Feb. 8, 1955 llllll W. A. ANDREWS ADJUSTABLE WAVE GUIDE TERMINATION Filed March 14, 1951 Zllrl 11111111111111 11/ III LIJJIII Ill/111111111111,

INVENTOR W/AL/AM. /4.fi/vp/ews ATTORNEY nited States PatentO ADJUSTABLE WAVE GUIDE TERMINATION William A. Andrews, Garden City, N. Y., assignor to The Sperry Corporation, a corporation of Delaware Application March 14, 1951, Serial No. 215,405

Claims. (Cl. 333-22) This invention relates to waveguide termination means and more particularly to such means of the adjustable type.

One of the primary problems in transmission line measurements is that of terminating the transmission line without producing an undesired reflected wave which will set up standing waves along the transmission line. Many prior art devices have been constructed for achieving this purpose. They generally comprise one or more pieces of resistive or lossy material which are inserted in the transmission line or waveguide and which are intended to completely absorb the energy in the guide in such a way as to prevent reflections. These lossy terminations are generally constructed in a form including a tapered energyincidence section for minimization of abrupt discontinuities to the incident electromagnetic energy. A great many devices of this type have been evolved with varying degrees of success and while many of them are quite eflicient, no material is a perfect absorber. There is always some small reflection even from the most carefully constructed terminations of the lossy type.

The present invention improves the characteristics and performance of a lossy termination by inserting a reflect ing obstacle which may be so adjusted to reflect a voltage in phase opposition to that of the residual reflection from the lossy termination. In other words, applicant realizes that there is a small unavoidable amount of reflection from a lossy type termination, since none of the known materials are perfect absorbers. To compensate for this, applicant inserts a reflecting obstacle. This would appear inconsistent with the absorption technique, but applicant has found that it is possible to adjust this reflecting obstacle so as to cancel out the reflection du to the lossy termination.

This invention may be described as consisting of a primarily resistive device in close combination with a reactive device, the latter being adjustable in magnitude and phase as related to the resistive device. The combined structure is adjustable as a whole in its phase relationship to an incident electromagnetic wave. This peculiar combination possesses new and unique advantages further described herein.

Accordingly, a principal object of the invention is to provide new and improved waveguide termination means.

Another object of the invention is to provide new and improved waveguide termination means having adjustable parameters.

Another object of the invention is to provide new and improved adjustable waveguide termination means including means to create a voltage in phase opposition to an undesired reflected voltage.

Another object of the present invention is to provide new and improved waveguide termination means including means to create a reflection cancellation voltage which is adjustable in phase and amplitude.

Another object of the present invention is to provide new and improved waveguide termination means including means to create a reflection cancellation voltage which is adjustable in phase and amplitude and means to move said corrected and reflection voltage along the guide.

Fig. 1a illustrates a sectional view of one form of the invention;

Fig. 1b is a cross-sectional view of the embodiment of Fig. la;

Fig. 2a illustrates a sectional view of an alternative embodiment of the invention; and

Fig. 2b is a sectional view of a portion of the embodiment of Fig. 2a.

Figs. 1a and 1b show longitudinal and cross sections respectively of a waveguide 1 which is terminated by a lossy type termination 2, 2. The termination may be in one piece or it may be constructed of several pieces and may comprise any of a number of materials which are known to have good absorption characteristics. Applicant has found that the material Transite is suitable in a particular embodiment. Other materials such as poly-iron, resistance card and cement-graphite combination, may be used as well as'other lossy materials common to the art such as are listed in microwave transmission design handbooks. The present invention is not limited to any particular absorbing material.

The lossy dielectric body 2, 2' is shaped to provide upper and lower long tapered ends, as shown in Fig. 1a to keep the net energy reflected from the body down to a reasonably low coetficient of reflection. The separate tapering edges need not be symmetrical; as shown they may be of unequal longitudinal extents, providing further diminution of the reflection coefficient. The termination 2, 2' is mounted on a metal plate 4, which, in turn, is connected to a hollow shaft 5. The hollow shaft 5 extends through the end of the guide 1 for a distance of at least one wavelength. The purpose of the hollow shaft is that the termination 2 may be moved axially along ihe guide 1 for reasons which will be more fully discussed ater.

The elements thus far described are believed to be more or less conventional in that the termination 2, 2' is of the well known absorption type, which applicant has found to be imperfect in that all the energy is not absorbed, and some small reflection remains. Applicant now inserts a reflecting obstacle 10, which is mounted on a rod 11, which extends along the longitudinal axis of the guide and through the hollow tube 5. The rod 11 may be rotatably and longitudinally moved with relation to the hollow tube 5 and the termination 2 by means of the knob 12. The rod 11 in one embodiment may be constructed of polystyrene although it is not limited to this material. It is adapted to be locked to the hollow tube 5 by means of the clamp 14. The rod 11 is preferably tapered at the end and the reflecting obstacle 10 is preferably of metal. The obstacle 10 is shown in Fig. la as a rectangular vane of thin metal.

The amount of reflection caused by the termination may be measured by voltage standing wave detector 20, Fig. la, which may be of a conventional type, and which is excited by the voltage within the guide by means of the probe 21. A conventional arrangement for measuring the standing wave ratio is to use a sliding detector mounted on a slotted section of transmission line. However, it will be shown hereinafter that in using the present invention, a stationary detector may be used to determine the standing wave ratio. Suitable standing wave detectors are discussed in the book Technique of Microwave Measurements, vol. 11, M. I. T. Series, McGraw-Hill, 1948, beginning on p. 490. The voltage detector for instance may comprise a crystal detector and a direct current meter. Any device which will indicate variations in the wave condition in the transmission line may be use The operation of the embodiment of Fig. 1a is as follows. Electromagnetic energy is received along the guide axis and a small amount of reflection will occur in the opposite direction due to the fact that the termination 2, 2' is not a perfect absorber. The reflecting obstacle 10 will also cause a reflection. The amplitude of the reflection from the obstacle 10 will be proportional to the angle of the obstacle 19 about the longitudinal axis of the guide. When the plane of the obstacle 10 is parallel to the direction of the electric vectors in the guide, there will be a maximum reflection, and when the plane of the obstacle 10 is rotated to be perpendicular to the electric vectors in the guide, there will be a minimum reflection. Therefore, the amplitude of the reflection due to the obstacle 10 may be varied by rotating the obstacle 10 about the longitudinal axis of the guide by means of the rod 11.

The phase of the reflection component due to the obstacle may be varied by altering the position of the obstacle 10 along the longitudinal axis of the guide, by means of the rod 11 and knob 12. Therefore, the obstacle 10 may be positioned such that the phase and amplitude of the reflection. component caused by: it may be adjusted to give a minimum standing wave. ratio reading on the voltmeter-detector 20.

After the initial adjustment to minimize the. standing wave ratio has been made by rotating. and sliding the obstacle 10, the position. of the obstacleflt). may. be fixed with relation to the termination 2, .2 by means of the clamp lock 14. Now the entire termination assembly including the lossy material 2, 2 and the obstacle 10. may be moved as a unit along the guide, by means of the hollow shaft 5. When this is done, if there is still: an appreciable standing wave existing along the guide, it will be indicated on the detectorZt) as the hollow shaft 5. is moved longitudinally with respect to the guide, and the voltmeter and probe 21 may be left in. a stationary position. Further adjustmentof the obstacle 10 may then be made. Applicant has found that this method of making this checking adjustment is much more sensitive than by conventional means.

The dimensions of the obstacle 10 are not critical because of the amplitude adjustment. The obstacle is not required to be very large in order to create a reflection voltage suflicient to balance the reflection fromthe termination 2, 2. In a. particular embodiment the length of the obstacle 10. has been 'made about one-half the smaller dimension of the guide 1 as shown in Fig. 1b. The width of the obstacle was made about one-half its length as shown in Fig. la. In higher power applications it may be desirable to reduce the length of the obstacle 10 in line with practices common to. the art and round off any corners so as to avoid sharp points which are susceptible to a voltage breakdown. For instance, an obstacle having a shape similar to an ellipsoid might be utilized. The obstacle 10 is not limited to a vane-type of construction but may have any construction-except perhaps that of a perfect sphere or a plate having a circular front view since there will be no amplitude variation when the sphere or circular plate is rotated. However, even a sphere may be used if it is mounted eccentrically to the guide axis and the rod 11.

The cross-sectional view of the invention as shown in Fig. 11) need not be limited to a rectangular guide as shown but may also comprise a square wave guide as known in the art or a rectangular wave guide oriented with its narrow side walls parallel to the plane of the reflecting obstacle 10.

Fig. 2a illustrates another embodiment of the invention which operates in exactly the same manner as that of Fig. la, but which is somewhat more refined in the details. of design. In Fig. 2a the reflecting obstacle 10' is composed of 4 parallel wires which are embedded in a polystyrene rod 11. The polystyrene rod 11 is narrowed down to a point just beyond the reflecting obstacle and near the open end of Wave guide 1 for the purpose OfminimiZing reflection due to the discontinuity caused by the change in dielectric constant occurring at the end of the rod 11. The lossy termination 2 is the same as that shown in Fig. 1a except that tapering edges are shown to be symmetrical. The lossy material 2 is fixed to a metal plate '4 which in turn is connected to two metal push rods 22, 23 which enter through holes in the endof waveguide 1. The push rods 22 and 23 are connected at their opposite ends to an end piece 24.

Fig. 2b shows details of a portion of Fig. 2a. The rod 11 is fixed at its end to a cylindrical end block 255 which is mounted for at least 90 of rotation about the axis of the rod in a slotted enclosing structure 26. There is a screw type clamp 27 which may be tightened to lock the relatively rotatable block 25 and enclosure 26 to prevent undesired rotation of the rod 11. Block 25 is fixed to rod 11 and enclosure 26 is fixed to rod 30. Therefore, rod 11 may be rotatably adjusted and then clamped by means of the knob of clamp 27.

A rod 3G is fixed to the enclosing structure 26, and extends through a hole in end piece 24 so that the rod 3t; is slidable with respect to the end piece 24, Therefore,

the polystyrene rod 11 and the obstacle 10" may be'moved longitudinally along the guide axis by handle 3;]; attached to the rod 34). Another screw type lock 32 is provided to lock rod 30 to the end piece 24, thus fixing; the position of obstacle 10' relative totermination 2.

The operation of the embodiment of Fig. 2a is exactly the same as that of Fig. 1a. The obstacle 10 may be rotated by end piece 25 by means of the knob on clamp 27, and the obstacle may be moved longitudinally by moving the handle 31 with respect to the end piece 24. When the obstacle 10' is properly positioned with respect to the termination 2, it may be locked in that position by tightening the screw type clamps 27 and 32 thereby fixing the relative position of the obstacle 1i) and the termination 2. The entire locked assembly including the obstacle 10 andthe termination 2 may now be moved relative to the guidev l. by the handle 31 which is now fixedly connected to. the end, piece 24 which in turn is fixedly connected to the termination 2 by means of the parallel rods 22 and 23. Therefore, the entire assembly slides on rods 22 and 23 relative to the waveguide 1.

The invention is specially adapted for use in the microwave measurements laboratory where power dissipation and impedance matchingv over a broadband are essential. When operated with suitable accessory equipment, the invention may be used with both C.-W. and modulated signals as:

(1) Pure resistive loads, adjusted. to give negligible power reflection, when obtaining. impedance data in the design of experimental components introduced. between the signal source and thepresent termination.

(.2) Standard matched loads which. will remain constant for comparison purposes when performing a. variety of microwave measurements.

Since many changes could be made in. the above construction and many apparently widely different embodiments of this invention could be made without departing from thescope thereof, it is intended that all matter contained in the. above description. or shown in. the accompanying drawings shallbe interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. Adjustable wave guide termination means comprising a wave guide having a longitudinal axis, absorbing means inserted within said guide, a reflective obstacle supported within said guide adjacent said absorbing means, the dimensionv of said obstacle as. measured along a vertical line perpendicular to said longitudinal axis being different from its dimension as measured along a horizontal line perpendicular to said axis, means connected to said obstacle for rotating said obstacle about said longitudinal axis to'vary the aspect presented. thereby to incident transverse electric waves, and means connected to said obstacle for sliding said obstacle along said longitudinal axis, said last two means comprising means for varying the amplitude and phase of reflections from said obstacle substantially independently of each other.

2. The apparatus as defined in claim 1' wherein said means connected to said obstacle to rotate said obstacle about said longitudinal axis. and said means connected to said obstacle to slide said obstacle along said. longitudinal axis comprises an axial rod;

3. The apparatus as defined in claim 2' wherein said absorbing means comprises a dielectric body having an axial bore therethrough, and wherein said axial rod extends through said. axial bore.

4. The apparatus. as defined in claimv 2 wherein said absorbing means and said axial rod are arranged to be clamped together for joint axial movement.

5. Adjustable waveguide termination means comprising a waveguide, a termination. of lossy material slidably mounted-within said guide, a reflective obstacle rotatably and. slidably supported within. said guide adjacent said lossy material, the length of said reflective obstacle in the vertical. plane being different from. the thickness of said reflective obstacle in the horizontal plane as viewed from one end of said. wave guide, means connected to said lossy material to slide said material along said guide parallel to the longitudinal axis thereof, means connected to said reflective obstacle to, rotate said obstacle about said: longitudinal axis for varying the magnitude of the reflection caused thereby. and means connected to saidobstacle to slide said obstacle along said longitudinal axis forvarying the phase of the reflection caused thereby, said obstacle moving means being adapted. to be locked relative to said lossy material moving means.

6. Adjustable waveguide termination means comprising a waveguide, a tapering termination slidably mounted within said guide, a reflective obstacle rotatably and slidably supported within said guide adjacent to said termination, the length of said reflective obstacle in the vertical plane being different from the thickness of said reflective obstacle in the horizontal plane as viewed from one end of said Wave guide, hollow shaft means connected to slide said termination along said guide parallel to the longitudinal axis thereof, and second shaft means mounted coaxially within said hollow shaft means and connected to rotate said obstacle about the longitudinal axis of said guide for varying the magnitude of the reflection caused thereby, and to slide said obstacle along said longitudinal axis for varying the phase of the reflection caused thereby, said obstacle moving means being adapted to be locked relative to said hollow shaft means.

7. Adjustable wave guide termination means comprising a wave guide having a longitudinal axis, a body of lossy dielectric material terminating said guide, reflecting means supported within said guide adjacent said lossy material, the dimension of said reflecting means as measured along a vertical line perpendicular to said axis being diflerent from its dimension as measured along a horizontal line perpendicular to said axis, means connected to said reflecting means for rotating said reflecting means about said longitudinal axis to vary the aspect presented thereby to incdient transverse electric waves, and means connected to said reflecting means for sliding said reflecting means along said longitudinal axis, said last two means comprising means for varying the amplitude and phase of reflections from said reflecting means independently of each other.

8. Adjustable wave guide termination means comprising a wave guide having a longitudinal axis, a piece of lossy material mounted in said guide, a reflective obstacle supported within said guide along said longitudinal axis adjacent said piece of lossy material, the dimension of said reflective obstacle as measured along a vertical line perpendicular to said axis being different from its dimension as measured along a horizontal line perpendicular to said axis, and a dielectric rod connected to said obstacle for rotating said obstacle about said longitudinal axis to vary the aspect presented thereby to incident transverse electric waves and for sliding said obstacle along said longitudinal axis, said rod means varying the amplitude and phase of reflections from said obstacle substantially independently of each other.

9. Wave guide termination means comprising a wave guide having a longitudinal axis, an absorption means inserted therein, a reflective obstacle rotatably and slidably supported within said guide adjacent said absorption means, the dimension of said reflective obstacle as measured along a vertical line perpendicular to said longitudinal axis being diiferent from its dimension as measured along a horizontal line perpendicular to said axis, and means for varying the amplitude and phase of reflections from said obstacle substantially independently of each other comprising an adjustab1e support connected to and adapted for rotating said obstacle about said longitudinal axis to vary the aspect presented thereby to incident transverse electric waves and for sliding said obstacle along said longitudinal axis.

10. Wave guide termination means comprising a wave guide having a tapering absorption means inserted therein, a reflective obstacle rotatably and slidably supported along the longitudinal axis of said wave guide adjacent said absorption means, the dimension of said reflective obstacle as measured along a vertical line perpendicular to said longitudinal axis being difierent from its dimension as measured along a horizontal line perpendicular to said axis, and means for varying the amplitude and phase of reflections from said obstacle substantially independently of each other for cancelling reflections from said absorption means including a dielectric rod connected to and adapted for rotating said obstacle about said longitudinal axis to vary the aspect presented thereby to incident transverse electric waves and to slide said obstacle along said longitudinal axis.

References Cited in the file of this patent UNITED STATES PATENTS Re. 23,131 Webber June 28, 1949 2,403,289 Korman July 2, 1946 2,404,797 Hansen July 30, 1946 2,423,396 Linder July 1, 1947 2,464,277 Webber Mar. 15, 1949 2,514,544 Hansen July 11, 1950 2,530,248 Larson Nov. 14, 1950 2,567,210 Hupcey Sept. 11, 1951 OTHER REFERENCES Publication I, Microwave Transmission Circuits, edited by Regan, vol. 9 of Radiation Laboratory Series published May 21, 1948; pages 485-497 relied on. (Copy in Division 69.)

Publication II, Microwave Duplexers, edited by Sullin and Montgomery, vol. 14 of Radiation Laboratory Series published before April 6, 1948, page 401 relied on. (Copy in Library.) 

